In fact, since diesel engines generate particularly high proportions of particles, providing exhaust lines with particulate filtering means can be considered. These means hold the particles back with high filtering efficiencies of the order of 80%. The ceramic monolith marketed by the Corning Company or the cartridge with coiled ceramic fibers, such as that described in Patent Cooperation Treaty patent application WO-95/27,843 can be mentioned by way of examples.
The technical difficulty encountered in the development of particulate filters is that they have to be periodically regenerated by combustion of the soot deposit. This combustion sometimes occurs naturally when the temperature of the gases reaches by itself the level required to initiate oxidation of the particles. However, average operating conditions generally lead to temperatures that are too low to spontaneously initiate combustion of the particles. This then leads to clogging of the filter, which is detrimental to the engine efficiency and eventually threatens the running thereof. The filter then has to be regenerated artificially.
Many techniques have been developed to that end. These techniques can be based on changes in the running of the engine intake throttling, exhaust throttling, advanced injection lag, or the techniques can be linked with energy supply in the exhaust gases or at the level of the filter (resistor, burner, micro-wave, . . . ). It is then necessary to control these various devices by means of an outer control driven by a computer. Most often, the criterion taken into account for regeneration initiation is the back pressure in the exhaust line.
In order to facilitate regeneration of particulate filters, a different approach of chemical nature consists in adding to the fuel an additive, for example an organometallic additive, that is thereafter found on the soot deposit, which generally leads to a decrease in the ignition temperature and therefore to a regeneration frequency increase.
Examples of the products most commonly used as additives are copper, iron, cerium, and sodium. Studies show that, in the presence of such additives, partial regenerations can occur spontaneously at relatively low exhaust gas temperatures (in the order of 200.degree. C.).
However, for certain driving modes, problems linked with the back pressure can subsist, so that an external power supply such as electric heating can be necessary.
Concerning power consumption, most of the well-known systems using electric heating perform a global heating of the filtering element. This leads to a high power consumption that is more or less controlled. Generally, the electrical power required to initiate total regeneration of the filter is high and often hardly compatible with the electrical resources on board the vehicle. European Patent EP-B1-0,485,179 illustrates a system based on this principle.
Besides, the regeneration conditions can greatly depend on the fouling condition of the filter.
French patent application EN.96/13,855 filed in the name of the applicant brings a solution to the problem of energy consumption of a regeneratable filtering set, whatever the running conditions of an engine.
According to this document, a filtering element is locally heated as a function of a global fouling measurement.
According to French patent application EN.96/11,292, the geometry of a filtering means is adapted as a function of predetermined strategies linked with the running of the engine.
In the field of fouling measurement in particulate filters, fouling detectors whose only purpose is to determine the degree of fouling of the filters are well-known.
The present invention overcome the aforementioned drawbacks and goes beyond the fouling measurement proper.